This invention relates to multidielectric mirrors and to coupling devices, in particular for use in a laser device. It also concerns optical devices comprising a multilayer mirror and a grating, said device being of high polarization selectivity, of particularly large structural tolerance, and of particularly large wavelength range.
A microchip laser polarization device is known from the market (Nanolase, Grenoble, France) whereby a mechanical stress induced transversally on a Nd:YAG microchip laser favours laser emission with the electric field polarized along the applied external force. The disadvantage of such solution is that it is a one by one solution applied to an otherwise batch production process of microlasers. Such solution is also limited because it can practically only lead to a uniform distribution of the electric field.
Another device is known from the scientific literature (V. N. Bel""tyugov, S. G. Protsenko, Y. V. Troitsky, xe2x80x9cPolarizing laser mirrors for normal light incidence xe2x80x9d, Proc. SPIE, Vol. 1782, 1992, p. 206) comprising a multilayer mirror composed of at least one corrugated interface between layers whereby the grating couples the undesired polarization of a gas laser into a guided mode of the multilayer, and induces a differential loss in the laser cavity between the coupled undesired polarization and the uncoupled desired polarization.
The practical limitations of this device are following.
In case one interface only is corrugated, the coupling efficiency into a guided mode of the multilayer is too weak for the device to be applied to microchip lasers where the beam diameter is 100 xcexcm or less; furthermore, the grating would in such case diffract the lasing polarization into diffraction orders propagating into the high index active crystal substrate. This results in non-acceptable losses. Moreover, the weakness of the coupling efficiency has the first consequence that the linewidth of the coupling phenomenon is very narrow and prevents the polarizing effect to be effective over a wide wavelength range, as for instance over the full gain bandwidth of about one nanometer of Nd:YAG lasers; a second consequence is that the spectral position of the narrow line at which the desired polarising effect occurs is highly dependent on the multilayer characteristics, thus on the fluctuations of its manufacturing conditions, and on the environmental dependence of the refractive index of the layers especially on humidity and temperature. This renders the device of the state of the art unusable in practice since it would require individual post-trimming and temperature control. In case all, or a large number of interfaces are corrugated, the coupling efficiency is increased but these corrugations lead to a perturbation of the layer deposition conditions which will be even less reproducible and provoke scattering losses on the lasing polarization.
Accordingly, it is a general object of the present invention to provide a novel and useful grating device such as a polarizing mirror or a polarizing coupling device in which the above described problems are eliminated.
Another, more specific object of the invention is to provide tolerant coupling means comprizing a grating capable of damping one polarization of a laser beam over a large wavelength range while maintaining the resulting scattering at a low level.
A first embodiment of a device according to the invention is an optical device comprising a substrate, a multilayer mirror, a pair of low and high refractive index layers, and a corrugation grating in said high refractive index layer.
Such a device can be used as a laser coupler or as a coupler in a laser device.
The combination of the pair of low and high refractive index layer, and of the corrugation grating in said high refractive index layer, results in a reduction of the reflection coefficient of a first polarization, by means of a destructive interference in the multilayer mirror for this first polarization, with essentially no change of the reflection coefficient for the other (second) polarization.
A beam of light is directed toward the optical device according to the invention through said substrate. In other words, the light beam is incident from the substrate side. The beam then successively traverses the multilayer mirror and the pair of low and high refractive index layers. One polarisation of the beam is reflected as in the absence of the grating. The other polarisation is reflected differently due to the grating, which is placed or made on, or in, the last high index layer, for example at the air side.
The multilayer mirror reflects both polarizations equally. The polarizing function is performed by the grating substructure comprizing the said corrugated pair of low and high refractive index layers.
For a corrugation grating on the last layer to have a non-negligible effect on the lasing condition in the cavity, the polarization selection effect can not simply be the coupling of one polarization into a guided mode of the multilayer as disclosed in the state of the art, for example in the article by Bel""tyugov cited above. The device according to the invention teaches the use of an abnormal reflection from the last high index waveguide grating under normal incidence: that is, when the incident polarization is coupled into the last high index layer mode, the optical power is reflected back from the grating with a phaseshift of xcfx80. Thus, the reflection of the damped polarization is not damped by its coupling into a waveguide mode, but it is efficiently reflected back into the cavity with a xcfx80 phaseshift by virtue of the said abnormal reflection, thus giving rise to an appreciable degree of destructive interference in the multilayer mirror for the coupled polarization, and consequently inducing on the latter a significantly more efficient damping.
Those familiar to the art will not be tempted to place the grating on the side of the last layer of the multidielectric mirror, for example at the air side, because the field is much weaker there than in the first layers, and because, in case of use in a laser device it is believed that the coupling of the polarization to be damped into a waveguide mode nearly outside the laser resonator will hardly have an influence on the intra-cavity polarization lasing conditions.
The substrate can be a laser active material substrate, for example the active material of a microchip laser.
The first embodiment of the device according to the invention can be advantageously used at one side of a laser cavity. Its efficiency is so high that the necessary damping of the coupled polarization can be obtained with a grating of very small depth, thus leading to a reduced scattering.
The efficiency of the grating is high because the last high index layer, acting as a waveguide, concentrates the modal field in the said layer, and in particular in the grating zone. The high radiation efficiency of the grating reduces the quality factor of the wavelength resonance and gives rise to a large wavelength tolerance of the grating mode coupling.
The first embodiment of the invention being substantially lossless, the damped polarization is not necessarily filtered out. It can thus be a lossless polarization filter exhibiting two different reflection coefficients for the two incident polarisations.
The polarization device according to the above first embodiment offers the following specific advantages:
it prevents the diffraction of the uncoupled polarization into the substrate, in particular in the case of a microchip laser,
the grating causes little scattering since it is not at the substrate side; in the case of a laser, it is not used at an active medium side within the laser resonator but at the air side,
it can provide a substantial and controllable difference between the reflection coefficient of the two polarizations without inducing any power loss on the polarization having the smaller reflection coefficient.
it can induce the polarization selection of very narrow beams,
it has a particularly large spectral bandwidth.
The second embodiment of the invention concerns an optical mirror, comprising a multilayer mirror, a grating and a metal or metallized substrate or a metal coated substrate, the multilayer mirror being located between the substrate and the grating.
The reduction of the reflection coefficient of the damped polarization is achieved by means of the coupling of the latter to one of the lossy modes of said multilayer deposited onto said substrate. In a laser device, the multilayer is at the substrate side which is at the inside of the laser cavity.
Whereas the coupling of the damped polarization to a guided mode of the multilayer according to the state of the art (V. N. Bel""tyugov et al) leads to a narrow band coupling, the device according to the second embodiment of the invention leads to a broader band coupling since a guided mode of the air and metal bordered multilayer waveguide suffers large losses.
Coupling to a TM mode is preferred. However, coupling to a TE mode is also possible, although the coupling linewidth is narrower.
The parameters of a structure achieving high and wavelength tolerant absorption can be found out by means of an available diffraction modeling code. The parameters can for instance be the number of layers of the multilayer, its first and last layers (low or high index), the type of metal, the thickness of the metal film, the period and depth of the grating.
The specific advantages of the second embodiment of the invention for a laser mirror are the following ones:
the metal borded multilayer offers a larger number of possible TM modes, and new types of modes which do not exist in the coupler of the state of art: two plasmon modes and all modes of effective index smaller than the substrate index ns,
the propagation constants of the TE modes and TM modes are interleaved, thus the lasing TE polarization does not suffer losses,
a metal substrate is compatible with possible fluid cooling in high power applications,
the coupled mode loss, thus the polarization filtering bandwidth, can be adjusted through the choice of the proper metal,
the metal substrate or film is placed beyond the last layer of the multilayer which usually has a large number of layers. Therefore the field of the incident lasing beam at this location is close to zero.
The third embodiment of the invention concerns both a mirror and a coupler. Alike in the second embodiment, the multilayer is at the subtrate side facing the laser cavity, the grating being in the last layer. Here, the device comprizes the grating, a multilayer, and the substrate, the grating having a period such that the incident damped polarization is coupled to one of the first order leaky modes of the multilayer of the same polarization. Leaky modes are transverse electromagnetic field resonances with total reflection at the air side and high partial reflection at the substrate side. The leakage of power in such resonance causes the coupling into such mode to be broadband and tolerant. Since the propagation constants of leaky modes of the two orthogonal polarizations are interleaved, the structure achieving high and wavelength tolerant leakage for the damped polarization and essentially zero leakage for the lasing polarization can be found out by means of codes available on the market.
The structure parameters which can be adjusted are for instance the number of layers in the multilayer, and/or the type of its first and last layers (low or high index), and/or the depth and period of the grating, and/or the polarisation.
The specific advantages of this third embodiment are the following ones:
the grating period is relatively large, thus easier to fabricate, and is essentially unique and predetermined since the first order leaky mode of so large a multilayer waveguide has an effective index very close to that of the substrate,
the third embodiment can be used as both a laser mirror and a laser coupler,
the flux resistance of the third embodiment is large since the leaky mode field does not exhibit a large amplitude anywhere in the structure, and since the power loss mechanism is not of an absorptive nature.
Beyond the specific advantages of the three embodiments which have been listed above, the common advantages of all three embodiments of the invention are the following ones:
the multilayer mirror of the device according to the invention can be that of a standard laser mirror,
technologically, the grating fabrication step comes after the whole stack of layers have been deposited,
the polarization filtering is performed by a device which can be produced by batch planar technologies,
the grating device according to the invention can define linear polarization distributions which are different from rectilinear.
For widening the wavelength range over which the reflection coefficient for one polarization is decreased, a device according to any embodiment of the present invention is so designed that the grating causes a substantial fall of the quality factor of the coupled mode for the incident beam of he said polarization.